In recent years the light-emitting diode (LED) has become a commonplace device for a broad range of applications. In the visible range it may provide communication between an electronic device and the user. In the infrared and visible ranges it may have broad applications for communications. A LED may be used in an optical isolator for decoupling an input signal from an output. It may be used as a light source in xerographic type printers. In many such applications it is an important desideratum that the LED maintains a relatively high light output which is not degraded by external influences.
In many applications of LEDs, the LED is imbedded in a transparent plastic potting material such as an epoxy resin or the like. This may be done to protect the LED from an external environment, however, in many applications the transparent material is employed to better match the relatively high index of refraction of the semiconductor material forming the LED, and thereby enhance the amount of light extracted from the LED.
It has been found, however, that under some conditions the light output of a LED may be significantly degraded due to stresses on the LED exerted by the plastic potting material. LEDs made of AlGaAs are particularly susceptible to light output degradation from such external causes. This material is stress sensitive and a permanent degradation in light output may occur in some LEDs when operated while a mechanical stress is applied.
An epoxy potting material is typically molded at temperatures of 100.degree. C. or more. The resin thus cured becomes rigid and may exert large stresses on the LED chip when cooled to room temperature or below. An exemplary plastic potting material may have a coefficient of thermal expansion in the order of 90 to 100 ppm/.degree.C. An exemplary semiconductor chip may have a coefficient of thermal expansion of only about 7 to 8 ppm/.degree.C., or more than an order of magnitude less than the potting material. The metal lead on which the LED is mounted has a coefficient of thermal expansion somewhere between such materials, and typically much closer to the thermal expansion of the LED chip. For example, when copper is used for the lead, it has a thermal expansion of 16.5 ppm/.degree.C. around room temperature.
Because of such differences in coefficient of thermal expansion, the plastic material may exert a high stress on the LED, particularly when operated at low temperature. Thus, for example, LEDs may be tested at -30.degree. C. for some applications or as low as -55.degree. C. for military applications. Under such conditions, some LEDs may exhibit quite substantial permanent degradation of light output.
The stress level on the LED chips may be reduced by making the chips very thin. This moves the light emitting region close to the metal lead, the thermal expansion coefficient of which is much closer to that of the LED material than that of the potting material. LED chips may be made thinner than 100 micrometers, however, these are quite difficult to handle in high volume manufacturing. As an alternative, rather large area LED chips may be used to reduce the absolute stress level and improve degradation performance. This however, significantly increases the cost of the LED chips since the yield per wafer is concomitantly reduced.
Thus, it is desirable to provide a means for reducing stress level on a LED for minimizing light degradation due to thermally imposed stress.
Thus, in practice of this invention the led on which the LED chip is mounted is formed with a cavity in which the LED chip is mounted. The cavity is only slightly larger in area than the chip for effectively shielding the LED from thermal expansion (or contraction) stresses applied by the plastic potting material. This dramatically improves the light output performance of the LED without the complications caused by thin chips or the cost of large area chips. Improvement from 60% of the LED chips showing severe light output degradation to none with such degradation are readily achieved.
This invention significantly reduces mechanical and thermal stresses on the LEDs since the thermal expansion mismatch is greatly reduced by bringing the light emitting junction very close to the metal lead. When the LED is largely buried in the cavity it is isolated from most of the encapsulating plastic potting material, resulting in significant reduction of stresses. The LED may be in a tight cavity, or a deep cavity for shielding the LED from thermal expansion stress.
LEDs have previously been placed in recesses in metal leads for a very different purpose and even in such embodiments thermal stresses may degrade light output. The recesses referred to are employed for maximizing the light effectively extracted from the LED. The walls of the recess are in the form of a cone or generally parabolic surface around the LED so that light emitted from the sides of a transparent LED is reflected by the walls of the recess in a direction more or less perpendicular to the front face of the LED.
When such reflective recesses are filled with plastic molding material, thermal stress may still be sufficient to cause light output degradation. A reason for this is that the recesses ar large diameter and shallow relative to the dimensions of the LED, so that a LED may be placed in the recess reliably by automated manufacturing equipment. Such large and shallow reflective recesses have not avoided light output degradation.
For example, a LED chip in a recess having the same depth as the height or thickness of the LED chip and a diameter at the open end of 1.27 millimeters, showed thermal stress induced light output degradation as poor as when the chip was mounted on a flat metal surface. An approximate stress analysis showed that shear stress exerted on the chip by the plastic molding material was reduced to about 60% of the stress exerted when the chip was mounted on a flat metal plate. Regardless, essentially all such LEDs showed light output degradation when operated at low temperature.
Examples of such recesses are seen in U.S. Pat. Nos. 3,764,862 by Jankowski; 3,863,075 by Ironmonger, et al.; and 3,914,786 by Grossi.